Thanks for your very interesting and thoughtful post. I will come back to this, but let me begin with a simple question or observation.
I think Malebranche is trying to get at a particular distinction that you do not address in your post. It is this:
Aquinas is saying that belief is like doubt, suspicion, and opinion insofar as it does not "attain the perfection of clear sight." Malebranche might have said something much blunter, "belief is like believing without seeing, or believing blindly." Regardless, in both cases a distinction is made between the clear sight of knowledge (
scientia) and the less-than-clear sight of belief. I do not see this distinction appearing in your post, and yet I want to say that this represents one of the basic differences between philosophy and Christianity. Thoughts?
First let me say that I don't know Malebranche well enough to clearly define exactly what he means by something like "believing blindly". Thus I am proceeding based on the common thought that believing blindly means without any basis of reason or evidence. If Malebranche means something else by this phrase, I will rely upon you to correct me.
In the Question you cite from the Summa, Thomas is talking about belief in the context of the ways that we know things. More specifically he starts by defining what it means "to think". In the course of that discussion he mentions thinking as the action of the intellect and as the action of the "cogitative power".
Further, the action of the intellect is distinguished as applying to knowledge of universals which is arrived at apart from sensory experience and the cogitative power is mentioned as relating to knowledge of particulars based upon sensory experience.
Thus we have here two distinct ways of arriving at knowledge, that are geared at different types of knowledge. The intellect functions independently of the senses and is geared to the understanding of universals. The cogitative power mediates between sensory data and particular knowledge and the rational faculties.
It is worth pointing out, when Thomas first uses the phrase "the perfection of clear sight" in this question, he is speaking about the intellect and not upon sensory experience. Thus he is not talking about literally seeing. He is talking about arriving at certain intellectual knowledge. Certain knowledge of universals.
Here I suspect we already have a distinct difference between what Thomas is saying and what Malebranche is saying. Again I don't know Malebranche well enough to say definitively, but I would guess that he he thinks of certain knowledge quite differently than Thomas, and again, I would guess he may not even believe that Universals exist at all in the same way that Thomas does, let alone that they can be certainly known. Feel free to correct me if I am wrong on that point.
Now let us consider faith.
Faith is another means of knowing, in addition to intellection and the cogitative power. The difference between them, much like the difference between Philosophy and Theology, is primarily the source or the means by which they derive their data.
Faith, in the Christian sense, is fundamentally belief in the testimony of a person, namely God. Divine revelation is God's testimony, primarily about himself, but also about us, and to a lesser degree the world.
Most of the things that are central in Divine revelation (God's testimony about himself) are things that we cannot know by natural means. In other words, they can't be discovered by Philosophy.
The subject of Question 2, that you quote from is the definition of belief as "thinking with assent". Thomas holds that "thinking with assent" is the correct definition of "belief".
This already contradicts at leas the common understanding of "blind faith" or "believing blindly". The basic concept is that belief is a process of deliberation and thinking, based on assent (faith) to divine revelation.
Before I go further I'll also point out that Thomas' answer to object 1 contradicts the notion of believing blindly.
The first objection states that faith cannot be "thinking with assent" because thinking derives from a Latin word which implies a process of research and/or discourse and that faith is defined as "assent without research".
Thomas answers this objection by essentially saying that the word "research" is being used equivocally in the objection and that there are two different types of research. The natural research being described in the objection is a process of logical discourse that demonstrates the conclusion with certainty. This cannot be done with matters of faith because they are not discoverable from the natural world. Thus you can't reason to them from the natural world. He goes on to say that the type of research that is done with objects of faith is research into those things by which a man is induced to believe. In other words, the evidence that supports the faith claim. He specifically mentions, was it actually said by God? and is it attended by miracles?
Thus Thomas clearly does think that Faith and Belief involve seeking evidence.
Lastly on to what is meant by "thinking with assent." and Belief being similar to doubt, suspicion, and opining.
In talking about thinking Thomas gives three definitions. The first is a very broad sense of any action of the intellect. He basically says that this doesn't apply and leaves it out.
Second is the most pertinent to this discussion. He defines thinking as a deliberative process of the intellect where it considers ideas that it has not yet arrived at definitive knowledge of. The definitive knowledge is what he refers to as "the perfection of clear sight of truth".
Essentially he is saying, if you know something definitively because it has already been conclusively demonstrated to be true, then you are not thinking about it anymore. You know it.
This is further demonstrated by what he has to say about God and thinking. He essentially says that God does not "think" because thinking requires a process of movement from something not having form to taking form. God already knows everything. Nothing in his mind takes form, it is already there in fullness. Thus God does not think as human beings think.
Thomas goes on to say that "belief" is "thinking with assent" in this second way of thinking. When we believe we are considering in a deliberative process, and our process of thinking is guided by assent to God's testimony about himself, which is faith in the divine revelation. \
We do not already know with certainty, because the ideas in question have not been definitively and conclusively proven by logical demonstration.
This is where he draws the comparison with doubt, suspicion, and opining. All three of these things are a deliberative process of thought in which the person has not arrived at definitive knowledge.
In the case of doubt, a person is deliberating between two or more possibilities, with no inclination to favor one over the other.
In the case of suspicion, the person has an inclination to favor one of the options more than the others.
In the case of opining, the person desires one option to be true, but is afraid that it might not be.
In the case of belief, the person assents firmly to one option, based on trust in God's testimony, despite that option not being definitively proven by natural reason. Thus belief has this in common with scientia, that the knowledge is firmly assented to, and differs from scientia in that the knowledge has not been definitively demonstrated by natural means.
The main objection thus, to the phrase believing blindly, is that it implies belief without basis. To say that something has not been definitively proven by natural reason, does not mean there is no basis for it.
If all a person means by "blind belief" is that belief holds something to be true which has not been conclusively demonstrated. Then the meaning intended is correct, but I would tend to say that the wording is poorly chosen in that case, and lacks a degree of distinction that is to be desired.
One thinks of St. Paul's statement that now we see through a glass darkly, but then we shall see as we are seen.
It's not that we don't see at all, our faith is not blind. We simply don't have perfect sight.
Moreover, far from faith being blind, faith rightly used improves our sight. As Pope John Paul II said, faith and reason are two wings of a bird. The bird can't fly without both. Ultimately reason functions no better without faith than faith does without reason. We might as well talk about blind reason as about blind faith.
